Corrosion-resistant alloy steels



Patented Sept. 14, 1954 UNITED STATES ATENT' OFFICE CORROSION-RESISTANT ALLOY STEELS George Henry Botliam, Crawley, and John Frederick Lancaster and David Whitfield Outram Dawson, London, England, assignors to The A. P. V. Company Limited, London, England, a company of Great Britain No Drawing. Application July 3, 1952, Serial No. 297,186

Claims priority, application Great Britain July 5, 1951 which most readily combines with the said free cutting element.

The selection of free cutting element will vary but the invention is of especial value when, as is most usual, the free cutting element added is sulphur. The metal included to combine with the sulphur must be more readily capable of combining with the sulphur than any other metal present in the alloy. Suitable metals may add to them in the melting shop small quantities be selected by a consideration of known thermoof free cutting elements such as selenium, telludynamical data but it may be observed that manrium, phosphorus and sulphur but especially the ganese and zirconium which very readily comlatter. The addition of those elements, however, bine with sulphur to form acid soluble sulphides affects adversely to varying degrees the resistance are inevitably excluded. of the steel to corrosion. For example a stain- The alloys of this invention may therefore be less steel containing 26% chromium, 4% nickel arrived at by two routes. In one method the and 1% manganese may be rendered more liable alloy may comprise those elements which alloy to corrosion by the addition of 0.2% sulphur. to provide the basic structure of the alloy, e. g.

The decrease of corrosion resistance may be alloys of iron with chromium, nickel, molybdendue to the formation of metallic compounds with um or vanadium or mixtures thereof, the said the free cutting elements and in particular, when alloys being substantially free from any metal sulphur is the free cutting element used, mansuch as manganese or zirconium which would ganese sulphide is formed if manganese is prescombine with the free cutting element more ent in the alloy. Manganese sulphide is soluble readily than the said alloying metals. in dilute acid media and gives rise to sulphuretted 535 In the second method there may be added to hydrogen. This gas is known to have a depassian alloy a proportion of a metal which while not. vating efiect on stainless steel and may cause essential to the basic structure of the alloy neversuch alloys to become active in environments theless combines preferentially with the free where they are normally passive, as shown in cutting element. the following examples: 30 One embodiment of the invention consists of a Composition Solution rate (mgs.

loss per sq. deci- Mgclbtinametrg 1 l er dais 7) Ni, Mo, M s, 11 Y mg 0 percent percent percent perc ent percent gi plus 18.1 7.8 2.5 1.1 0. 34 Free 118 18.5 8.0 2.0 1.1 0.04 Difficult.- 10 18. 7.0 0.05 1.0 0.34 r 1,340 18. 7.9 0.05 1.1 0.03 890 21.7 5.2 1.6 1.0 0.28 400 22.3 4.8 1.0 0.9 0.03 2s. 4.4 0.05 1.0 0. 20 a, 200 26.3 4.9 0.3 0.9 0.01 Difficult... 285

It is a main object of the present invention to minimize the loss of resistance to corrosion of alloy steels, and in particular of stainless steels, to which have been added free cutting elements to render them readily machinable.

According to the present invention an alloy steel of easy machinability and high corrosion resistance comprises a free cutting element and includes within its composition a metal which combines with the said free cutting element to form an acid resistant compound or complex, said metal being, of those contained in the alloy, that stainless steel contains by weight 26% chromium, I 4% nickel, 0.2% sulphur and 2.5% copper, and

does not contain any substantial amount of any metal, for example manganese or zirconium, such as will combine with sulphur in the presence of copper.

The chemical tests carried out on a stainless steel to discover its resistance to corrosion often include immersion of the steel in nitric acid, and immersion in sodium chloride solution containing an organic acid such as lactic acid. Stainless steel containing 26% chromium, 4% nickel and 1% manganese (referred to above) gives satisfactory results in these tests, but with the addition of 0.2% sulphur to render it readily machinable it gives Very poor results. It is believed that, at high temperatures, for instance about 1500 C., the sulphur combines with the manganese to form manganese sulphide which, dispersed in fine particles through the steel, promotes the breaking up of swarf in machining. The manganese sulphide is, however, soluble in dilute nitric and hydrochloric acids, and on going into solution causes corrosion of the steel.

The hydrogen sulphide which is formed further acts as a depassivator.

The replacement, in the steel previously mentioned containing 26% chromium, 4% nickel and 1% manganese, of the manganese by, for example, 2.5% copper causes a much smaller loss of resistance to corrosion on the addition of sulphur. It is believed that the sulphur combines with the copper to form copper sulphide which is insoluble in dilute acids. Experiments have shown, and known data of free energy of formation of sulphides confirm, that, at the pouring temperature, the order of preferential combination with sulphur will be-manganese; copper; iron and chromium. Accordingly a steel containing copper to minimize the loss of resistance to corrosion must have the copper in a quantity exceeding that required to combine with the sulphur (i. e. a ratio exceeding 5:1) and must not contain substantial amounts of any metal, for example manganese or zirconium such as to combine with the sulphur in preference to copper.

A large excess of copper further assists the machinability of the stainless steel.

In alloy steels formulated according to this 4 and C, a 10% solution of sulphuric acid at room temperature.

Constitution of Alloy No A B C Cr Ni S Mn Cu It will be seen from the foregoing table that the corrosion resistance of alloy No. 1 containing manganese is considerably reduced by the addition of sulphur as free cutting element (alloy No. 2). However, by the addition of copper and the virtual exclusion of manganese, alloy No. 3 is obtained which has better corrosion resistance to all treatments than alloys Nos. 1 or 2. This improved resistance is however substantially reduced by the reinclusion of manganese as shown by alloy No. 4. A further improvement obtained in alloy No. .5 by the exclusion of manganese and the presence of a more substantial proportion of copper, and improved resistance in alloy No. 6 as compared with alloy No. 2 is obtained merely by the omission of the manganese, no copper being added.

Similarly improved resistance may be obtained if the alloy contains molybdenum or vanadium. These elements form with sulphur, sulphides having a greater resistance to acid corrosion than iron sulphide and .as they are formed preferentially at the pouring temperature they constitute further examples in accordance with the invention.

The following table shows the influence on corrosion resistance of the addition of manganese to a sulphur bearing 18% chromium, 8% nickel, 2.5% molybdenum, cast stainless steel and it will be noted that on comparing alloys I and III, the advantages of free machining have been obtained without reduction of the corrosion resistance of steel I, such as is shown in the case of alloy II.

Analysis Solution Rate Alloy Boilin g v Mark Cr, Per- Ni, Per- Mo, Per- Mn, Pers, Per- 1% Lactic 10% cold cent cent cent cent cent Acid-121% Acid H1801 18. 5 8. l) 2. 6 1. l 0. 04 16 1 23 18. 1 7. 8 2. 5 1. l 0. 34 118 13 63 18. 1 8. O 2. 3 O. 1 0.32 l '1 Nil invention and which do not contain copper, manganese or zirconium, the acid-insoluble separate phase responsible for the easy breaking of swarf may be a complex of chromium and iron sulphides such as Fe(CrS2) 2, which is known to be substantially unaffected by dilute acids.

Alloy steels of the present invention contain 21% to 29% chromium, 4% to 6% nickel, 0.10% to 0.75% sulphur, and a manganese content not greater than 0.4%.

The following table shows the improvement in results obtained with alloys formulated in accordance with the present invention. The figures given under A, B and C are respectively the loss in milligrammes per sq. decimeter per day for; A, an aqueous solution containing 1% lactic acid and 1% sodium chloride at boiling point, B, a 1% aqueous solution of nitric acid at boiling point,

It should be noted that in the case of alloys to composition II, considerable variations in corrosion resistance are encountered. The figure given in the above table for alloy 11 in boiling lactic acid plus sodium chloride has on occasion been found to be as high as 780. One of the advantages to be gained by the elimination of manganese as far as is practicable is that the variation of corrosion resistance of a given composi tion is greatly reduced. Experimental work suggests that when the sulphur content is about 0.3% the manganese should not be greater than 0.4% but is preferably below 0.2%.

It follows from the foregoing description that an improved corrosion resistance in steels containing chromium, nickel, molybdenum or vanadium, singly or together, and which contain sulphur as the free cutting element, is achieved by 1. Alloy steels consisting of iron alloyed with 10 21-29% chromium, l-6% nickel, (MO-0.75% sulphur and wherein the manganese content is not greater than 0.4% and preferably below 0.2%.

2. Alloy steels consisting of iron alloyed with 21-29% chromium, 46% nickel, Gill-0.75% sulphur and a proportion of copper relative to sulphur of at least 5:1 and wherein the manganese content is not greater than 0.4%, preferably below 0.2%.

References Cited in the file of this patent UNITED STATES PATENTS Name Date Palmer June 5, 1934 OTHER REFERENCES Stainless Iron and Steel, Second and Revised Edition, pages 249 and 250, edited by Monyhenny, published in 1931 by Chapman and Hall, Limited,

Number 5 London, England. 

2. ALLOY STEELS CONSISTING OF IRON ALLOYED WITH 21-29% CHROMIUM, 4-6% NICKEL, 0.10-0.75% SULPHUR AND A PROPORTION OF COPPER RELATIVE TO SULPHUR OF AT LEAST 5:1 AND WHEREIN THE MANGANESE CONTENT IS NOT GREATER THAN 0.4%, PREFERABLY BELOW 0.2%. 